U.S. patent number 9,804,047 [Application Number 14/989,709] was granted by the patent office on 2017-10-31 for integrated pressure sensor with double measuring scale, pressure measuring device including the integrated pressure sensor, braking system, and method of measuring a pressure using the integrated pressure sensor.
This patent grant is currently assigned to STMicroelectronics S.r.l.. The grantee listed for this patent is STMICROELECTRONICS S.R.L.. Invention is credited to Daniele Caltabiano, Marco Ferrera, Domenico Giusti, Bruno Murari, Alberto Pagani.
United States Patent |
9,804,047 |
Pagani , et al. |
October 31, 2017 |
Integrated pressure sensor with double measuring scale, pressure
measuring device including the integrated pressure sensor, braking
system, and method of measuring a pressure using the integrated
pressure sensor
Abstract
A pressure sensor with double measuring scale includes: a
flexible body designed to undergo deflection as a function of a the
pressure; piezoresistive transducers for detecting the deflection;
a first focusing region designed to concentrate, during a first
operating condition, a first value of the pressure in a first
portion of the flexible body so as to generate a deflection of the
first portion of the flexible body; and a second focusing region
designed to concentrate, during a second operating condition, a
second value of said pressure in a second portion of the flexible
body so as to generate a deflection of the second portion of the
flexible body. The piezoresistive transducers correlate the
deflection of the first portion of the flexible body to the first
pressure value and the deflection of the second portion of the
flexible body to the second pressure value.
Inventors: |
Pagani; Alberto (Nova Milanese,
IT), Murari; Bruno (Monza, IT), Ferrera;
Marco (Concorezzo, IT), Giusti; Domenico (Monza,
IT), Caltabiano; Daniele (Agrate Brianza,
IT) |
Applicant: |
Name |
City |
State |
Country |
Type |
STMICROELECTRONICS S.R.L. |
Agrate Brianza |
N/A |
IT |
|
|
Assignee: |
STMicroelectronics S.r.l.
(Agrate Brianza, IT)
|
Family
ID: |
53765468 |
Appl.
No.: |
14/989,709 |
Filed: |
January 6, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20160349129 A1 |
Dec 1, 2016 |
|
Foreign Application Priority Data
|
|
|
|
|
May 27, 2015 [IT] |
|
|
102015000018354 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01L
15/00 (20130101); G01L 9/0054 (20130101); G01L
9/0052 (20130101); G01L 9/0045 (20130101) |
Current International
Class: |
G01L
9/06 (20060101); G01L 9/00 (20060101); G01L
15/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Allen; Andre
Attorney, Agent or Firm: Seed IP Law Group LLP
Claims
The invention claimed is:
1. A pressure sensor with double measuring scale, configured to
receive a pressure acting in a direction, comprising: a flexible
body configured to undergo, at least in part, deflection as a
function of said pressure; a first substrate arranged facing a
first side of the flexible body; a transduction assembly configured
to generate a first output signal and a second output signal as a
function of deflections of respective first and second portions of
the flexible body; a first focusing region arranged between the
first substrate and the flexible body and configured to
concentrate, during a first operating condition, a first pressure
value of said pressure in the first portion of the flexible body
and generate a deflection of only the first portion of the flexible
body; and a second focusing region arranged between the first
substrate and the flexible body and configured to concentrate,
during a second operating condition, a second pressure value of
said pressure in a second portion of the flexible body and generate
a deflection of the second portion of the flexible body, the
transduction assembly being configured to generate the first output
signal during the first operating condition by correlating the
deflection of the first portion of the flexible body to the first
pressure value, and to generate the second output signal during the
second operating condition by correlating the deflection of the
second portion of the flexible body to the second pressure
value.
2. The pressure sensor according to claim 1, further comprising a
cavity between the first substrate and the flexible body, wherein
the first focusing region extends in the cavity and has a first
thickness equal to a distance existing between the first substrate
and the flexible body, and the second focusing region extends in
the cavity and has a second thickness smaller than said first
thickness.
3. The pressure sensor according to claim 1, wherein: the second
focusing regions protrude from the first substrate and surround, at
least in part, the first focusing region, and the first substrate
is configured to undergo deflection in the direction of application
of the pressure, and the second focusing regions are configured to
trigger passage from the first operating condition to the second
operating condition by contacting the flexible body.
4. The pressure sensor according to claim 1, further comprising: a
second substrate facing a second side, opposite to the first side,
of the flexible body; and spacers extending between the second
substrate and the flexible body, defining a cavity, on which the
flexible body is suspended, so that the flexible body forms a
membrane configured to undergo deflection during the second
operating condition.
5. The pressure sensor according to claim 1, further comprising a
buried first cavity extending within said flexible body, directly
facing the first portion of the flexible body in such a way that
the first portion of the flexible body extends suspended on the
buried first cavity to form a membrane.
6. The pressure sensor according to claim 5, further comprising a
second cavity between the first substrate and the flexible body,
wherein the first focusing region extends in the second cavity and
has a first thickness equal to a distance existing between the
first substrate and the flexible body, and the second focusing
region extends in the second cavity and has a second thickness
smaller than said first thickness, wherein the thickness of the
second focusing regions in said direction, and a dimension of the
buried first cavity in said direction, are chosen in such a way
that the membrane comes into contact with a bottom of the buried
first cavity upon contact between the second focusing regions and
the flexible body.
7. The pressure sensor according to claim 6, wherein the dimension
of the buried first cavity in the direction of action of the
pressure is equal to a corresponding dimension of the second cavity
at the second focusing regions.
8. The pressure sensor according to claim 5, wherein said
transduction assembly comprises first piezoresistive elements
arranged, at least in part, in said membrane.
9. The pressure sensor according to claim 5, wherein said
transduction assembly further comprises second piezoresistive
elements arranged in the second portion of the flexible body,
outside said membrane.
10. The pressure sensor according to claim 9, wherein said second
portion of the flexible body that houses the second piezoresistive
elements is a solid and compact region of the flexible body.
11. The pressure sensor according to claim 9, wherein the first
piezoresistive elements are electrically connected together in a
first Wheatstone-bridge circuit, or a first ring oscillator
circuit, configured to supply the first output signal; and the
second piezoresistive elements are electrically connected together
in a second Wheatstone-bridge circuit, or a second ring oscillator
circuit, configured to supply the second output signal.
12. The pressure sensor according to claim 5, wherein the membrane
has a thickness in the direction of action of the pressure between
1 .mu.m and 60 .mu.m.
13. The pressure sensor according to claim 1, wherein said first
pressure value is smaller than the second pressure value, in
particular the first pressure value is in a range 0-20 N, and said
second pressure value is higher than 20 N.
14. The pressure sensor according to claim 1, wherein the flexible
body includes: a monolithic region having a thickness at the second
portion along the direction of action of the pressure, between 50
.mu.m and 900 .mu.m; and an interface layer, extending between the
monolithic region and the first and second focusing regions,
configured to distribute said pressure uniformly over said
monolithic region.
15. The pressure sensor according to claim 1, further comprising a
communication interface configured to enable communication of the
first output signal and/or second output signal to an external read
circuit, the communication interface including a first inductor
winding integrated in the flexible body and a second inductor
winding integrated in the first substrate.
16. The pressure sensor according to claim 1, further comprising a
first winding integrated in the flexible body and configured to
inductively couple with a second winding of a printed-circuit board
external to said pressure sensor, to form a interface for
communication of the first output signal and/or the second output
signal to said printed circuit board.
17. A pressure measuring device, comprising: a measuring circuit;
and a sensor assembly electrically coupled to the measuring circuit
and including a pressure sensor with double measuring scale, the
pressure sensor being configured to receive a pressure acting in a
direction and including: a flexible body configured to undergo, at
least in part, deflection as a function of said pressure; a first
substrate arranged facing a first side of the flexible body; a
transduction assembly configured to generate a first output signal
and a second output signal as a function of deflections of
respective first and second portions of the flexible body; a first
focusing region arranged between the first substrate and the
flexible body and configured to concentrate, during a first
operating condition, a first pressure value of said pressure in the
first portion of the flexible body and generate a deflection of
only the first portion of the flexible body; and a second focusing
region arranged between the first substrate and the flexible body
and configured to concentrate, during a second operating condition,
a second pressure value of said pressure in a second portion of the
flexible body and generate a deflection of the second portion of
the flexible body, the transduction assembly being configured to
generate the first output signal during the first operating
condition by correlating the deflection of the first portion of the
flexible body to the first pressure value, and to generate the
second output signal during the second operating condition by
correlating the deflection of the second portion of the flexible
body to the second pressure value.
18. The device according to claim 17, wherein said measuring
circuit is integrated in the flexible body of the pressure
sensor.
19. A system, comprising: a pressure actuator configured to produce
a pressure acting in a direction; and a pressure measuring device
that includes a measuring circuit and a pressure sensor with double
measuring scale, the pressure sensor being configured to receive
the pressure acting in the direction and including: a flexible body
configured to undergo, at least in part, deflection as a function
of said pressure; a first substrate arranged facing a first side of
the flexible body; a transduction assembly configured to generate a
first output signal and a second output signal as a function of
deflections of respective first and second portions of the flexible
body; a first focusing region arranged between the first substrate
and the flexible body and configured to concentrate, during a first
operating condition, a first pressure value of said pressure in the
first portion of the flexible body and generate a deflection of
only the first portion of the flexible body; and a second focusing
region arranged between the first substrate and the flexible body
and configured to concentrate, during a second operating condition,
a second pressure value of said pressure in a second portion of the
flexible body and generate a deflection of the second portion of
the flexible body, the transduction assembly being configured to
generate the first output signal during the first operating
condition by correlating the deflection of the first portion of the
flexible body to the first pressure value, and to generate the
second output signal during the second operating condition by
correlating the deflection of the second portion of the flexible
body to the second pressure value.
20. The system according to claim 19, comprising: a brake, wherein
the measuring circuit is configured to generate a control signal
and the pressure actuator includes an electromechanical actuator
configured to exert a braking action on said brake in response to
the control signal generated by the measuring circuit.
21. A method, comprising: measuring a pressure by a pressure sensor
with double measuring scale provided with: a flexible body
configured to undergo, at least in part, deflection as a function
of said pressure; a first substrate arranged facing a first side of
the flexible body; a transduction assembly configured to generate a
first output signal and a second output signal as a function of the
deflection of the flexible body; a first focusing region; and a
second focusing region, the measuring including: applying, during a
first operating condition, a first pressure value of said pressure
to said pressure sensor; concentrating, through the first focusing
region, the first pressure value at a first portion of the flexible
body and generate a deflection of the first portion of the flexible
body; applying, during a second operating condition, a second
pressure value of said pressure to said pressure sensor;
concentrating, through the second focusing region, the second
pressure value at a second portion of the flexible body and
generate a deflection of the second portion of the flexible body;
correlating, by the transduction assembly, the deflection of the
first portion of the flexible body to the first pressure value
during the first operating condition; generating, the first output
signal as a function of the deflection of the first portion of the
flexible body; correlating, by the transduction assembly, the
deflection of the second portion of the flexible body to the second
pressure value during the second operating condition; and
generating, the second output signal as a function of the
deflection of the second portion of the flexible body.
22. The method according to claim 21, wherein the pressure sensor
further comprises a first cavity, between the first substrate and
the flexible body; and a buried second cavity, in said flexible
body, that directly faces the first portion of the flexible body in
such a way that the first portion of the flexible body extends
suspended on the buried second cavity to form a first membrane, the
method further comprising passing from the first operating
condition to the second operating condition at a pressure value of
said pressure that brings the first membrane into contact with a
bottom of the buried pressure cavity.
23. The method according to claim 21, wherein: the pressure sensor
further comprises: a second substrate facing a second side,
opposite to the first side, of the flexible body; and spacers
extending between the second substrate and the flexible body, which
define a third cavity, the flexible body being suspended over the
third cavity so as to form a second membrane, concentrating the
second value of said pressure in the second portion of the flexible
body includes causing a deflection of the flexible body towards an
inside of the third cavity.
Description
BACKGROUND
Technical Field
The present disclosure relates to an integrated pressure sensor
with double measuring scale, to a pressure measuring device
including the integrated pressure sensor, to a braking system, and
to a method of measuring a pressure that uses the integrated
pressure sensor. In particular, the ensuing treatment will make
explicit reference, without this implying any loss of generality,
to use of said pressure sensor in a braking system of a vehicle, in
particular an electromechanical braking system of the BbW
(Brake-by-Wire) type.
Description of the Related Art
As is known, disk-brake systems of a traditional type for vehicles
comprise a disk fixed with respect to a respective wheel of the
vehicle, a calliper associated with the disk, and a hydraulic
control circuit. The calliper houses within it pads of friction
material, and one or more pistons connected to the hydraulic
control circuit. Following upon an action, exerted by a user of the
vehicle, on the brake pedal, a pump in the hydraulic control
circuit pressurizes a fluid contained in the circuit itself.
Consequently, the pistons, equipped with purposely provided sealing
elements, come out of respective seats and come to press the pads
against the surface of the disk, in this way exerting a braking
action on the wheel.
Recently, so-called DbW (Drive-by-Wire) systems have been proposed,
which envisage electronic control of the main functions of a
vehicle, for example the steering system, the clutch, and the
braking system. In particular, electronically controlled braking
systems have been proposed, which envisage replacement of the
hydraulic callipers with actuators of an electromechanical type. In
detail, appropriate sensors detect operation of the brake pedal and
generate corresponding electrical signals, which are received and
interpreted by an electronic control unit. The electronic control
unit then controls intervention of the electromechanical actuators
(for example, pistons driven by an electric motor), which exert the
braking action on the corresponding brake disks, through the pads.
The electronic control unit further receives from sensors
associated to the braking system information on the braking action
exerted by the electromechanical actuators so as to provide an
appropriate closed-loop feedback control, for example, via a PID
(Proportional-Integral-Derivative) controller. In particular, the
electronic control unit receives information on the pressure
exerted by each actuator on the respective brake disk.
To measure the aforesaid pressure, pressure sensors are used with
high sensitivity both at low pressures and at high pressures, and
likewise with a high full-scale value. In fact, there is
particularly felt the need to measure pressure with a double
measuring scale in order to measure both low pressures and high
pressures with high precision. Furthermore, the force with which
the pads are pressed against the disk may assume values from 0 N up
to a maximum comprised in the range 10,000 to 35,000 N, according
to the braking system.
There are currently known sensors capable of measuring high
pressure values, which are made with a steel core, fixed on which
are strain-gauge elements.
The strain-gauge elements detect the geometrical deformation of the
core to which they are associated by variations of electrical
resistance. However, these sensors, for reasons of reliability,
size, and costs may be applied and used only for characterization
and development of a braking system of the type described
previously, but not in the production stage. Furthermore, they do
not have a high precision and have only one measuring scale.
Likewise known are integrated pressure sensors, obtained with
semiconductor technology. These sensors typically comprise a thin
membrane suspended over a cavity made in a silicon body. Diffused
within the membrane are piezoresistive elements connected together
to form a Wheatstone bridge. When subjected to a pressure, the
membrane undergoes deformation, causing a variation of resistance
of the piezoresistive elements, and thus an unbalancing of the
Wheatstone bridge. However, such sensors may not be used for
measurement of high pressures, in so far as they have low
full-scale values (namely, in the region of 10 kg/cm.sup.2), in
particular considerably lower than the pressure values that are
generated in the braking systems described previously.
A solution to the aforementioned problems is disclosed by U.S. Pat.
No. 7,578,196, where, for measurement of high pressures, a membrane
sensor is proposed provided with first piezoresistive elements, set
in the proximity of the membrane, and second piezoresistive
elements, set at a distance from the membrane, in a bulk area that
is solid and compact. The first piezoresistive elements are
designed to detect a deflection of the membrane that undergoes
deformation under the action of low pressures, until a maximum
deflection (saturation) is reached. The second piezoresistive
elements are designed to detect a stress of a transverse type (but
not longitudinal, in so far as there is no bending or phenomena of
curving of the bulk area) that acts on the second piezoresistive
elements as a result of an increase in pressure beyond the
saturation pressure of the membrane.
This type of sensor provides a good accuracy of measurement at low
pressures (signal supplied by the first piezoresistive elements),
but a poor accuracy at high pressures (signal supplied by the
second piezoresistive elements). Furthermore, this type of sensor
does not discriminate between pressure variations lower than a
minimum detection threshold.
For the feedback-control system of the braking system to function
optimally, it is expedient also for the measurements made at high
pressures to be accurate and sensitive to minimal pressure
variations.
BRIEF SUMMARY
Some embodiments of the present disclosure are a pressure sensor, a
pressure measuring device, including the integrated pressure
sensor, and a method of measuring a pressure that uses the pressure
sensor which will enable the aforementioned disadvantages and
problems to be overcome and in particular will present a double
measuring scale, a high full-scale value, and high accuracy and
sensitivity, so as to measure both high pressures and low pressures
with a good level of precision.
According to the present disclosure, an integrated pressure sensor
with double measuring scale, a pressure measuring device including
the integrated pressure sensor, a braking system, and a method of
measuring a pressure that uses the integrated pressure sensor are
consequently provided as defined in the annexed claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
For a better understanding of the present disclosure, preferred
embodiments thereof are now described, purely by way of
non-limiting example and with reference to the attached drawings,
wherein:
FIG. 1 illustrates a block diagram of a braking system of a
brake-by-wire electromechanical type;
FIG. 2 is a cross-sectional view (not in scale) of a pressure
sensor obtained according to one embodiment of the present
disclosure;
FIG. 3 shows a Wheatstone-bridge circuit formed by piezoresistive
elements, integrated in the pressure sensor of FIG. 2;
FIG. 4 is a perspective view of the pressure sensor of FIG. 2;
FIG. 5 is a cross-sectional view (not in scale) of a pressure
sensor obtained according to a further embodiment of the present
disclosure;
FIG. 6 is a cross-sectional view (not in scale) of a pressure
sensor obtained according to a further embodiment of the present
disclosure;
FIG. 7 shows an electrical signal generated at output from the
pressure sensor of FIG. 6 as a function of a pressure to which the
sensor itself is subjected in use; and
FIGS. 8-12 show respective embodiments of pressure sensors designed
to transfer electrical signals, generated as a function of a
pressure to which the sensor is subject in use, at output from the
pressure sensor.
DETAILED DESCRIPTION
FIG. 1 shows an example of block diagram of a braking system 1 of
an electromechanical type (a so-called "brake-by-wire" system),
comprising: a brake pedal 2; first sensors 3 designed to detect the
travel C and speed of actuation v of the brake pedal 2; an
electronic control unit 4, connected to the first sensors 3; an
electromechanical actuator 5 connected to the electronic control
unit 4, and constituted by an electric motor 6 and by a piston 7,
which is connected to the electric motor 6 for example by a
connection element of the wormscrew type (not illustrated); a brake
disk 8, connected to the electromechanical actuator 5 and fixed
with respect to a wheel of a vehicle (in a way known and not
illustrated); and second sensors 9, which are designed to detect
information regarding the braking action exerted by the
electromechanical actuator 5 on the brake disk 8 and are
feedback-connected to the electronic control unit 4.
In use, the first sensors 3 send data regarding the travel C and
speed of actuation v of the brake pedal 2 to the electronic control
unit 4, which, as a function of said data, generates a control
signal (in voltage V or current I) for the electromechanical
actuator 5 (in particular, for the electric motor 6). As a function
of said control signal, the electric motor 6 generates a torque
that is converted into a linear movement of the piston 7 by the
connection element of the wormscrew type. Consequently, the piston
7 presses on the brake disk 8 (via pads of abrasive material, not
illustrated) so as to brake rotation thereof. The second sensors 9
detect the value of the pressure P exerted by the piston 7 on the
brake disk 8 and the position x of the piston 7 with respect to the
brake disk 8, and send said feedback data to the electronic control
unit 4. The electronic control unit 4 thus implements a closed-loop
control (for example, a PID control) of the braking action.
According to one aspect of the present disclosure, the second
sensors 9 comprise a pressure sensor according to any one of the
embodiments described in what follows, in particular of an
integrated type, which are obtained in MEMS technology and are
designed to measure the pressure P exerted by the piston 7 on the
brake disk 8. In a way not illustrated, the pressure sensor 15 is
housed in a casing of the electromechanical actuator 5 and is
configured in such a way as to be sensitive to the pressure P
exerted by the piston 7.
In detail, as represented in FIG. 2 in a reference system of
orthogonal axes X, Y, Z, the pressure sensor 15 comprises a
monolithic body 16 of semiconductor material, preferably silicon,
in particular monocrystalline silicon, for example of an N type
with orientation (100) of the crystallographic plane. The
monolithic body 16 has a quadrangular cross-section, for example
square of side 1 (measured along the axis X or Y) comprised, for
example, between 1 mm and 20 mm, preferably between 10 mm and 15
mm, delimited at the top by a first surface 16a and at the bottom
by a second surface 16b, which is opposite and parallel to the
first surface 16a. The monolithic body 16 has a thickness, measured
along Z between the first and second surfaces 16a, 16b, equal to w
and substantially uniform, for example comprised between 50 and 900
.mu.m, preferably comprised between 500 and 900 .mu.m, in
particular equal to 720 .mu.m.
The monolithic body 16 comprises a bulk region 17 and a first
cavity 18, buried in the monolithic body 16. The first cavity 18
has a cross-section that is, for example, square with side
comprised between 300 and 400 .mu.m and a thickness (measured along
the axis Z) comprised between 2 and 6 .mu.m, for example 4 .mu.m.
The first cavity 18 is separated from the first surface 16a by a
thin portion of the monolithic body 16, which forms a membrane 19,
which has a thickness comprised, for example, between 1 and 60
.mu.m, preferably between 4 and 10 .mu.m. The bulk region 17 is
thus the portion of the monolithic body 16 that surrounds the
membrane 19 and the first cavity 18.
The membrane 19 is flexible and is able to undergo deflection in
the presence of external loads. In particular, as described in
detail hereinafter, the membrane 19 undergoes deformation as a
function of a force or pressure P acting on the monolithic body 16.
According to one embodiment, the thickness of the membrane 19 is
smaller than the thickness of the first cavity 18 in order to
prevent shear stresses in the points of constraint of the membrane
19, which might cause failure of the membrane itself.
The first cavity 18 may be obtained with the manufacturing process
described in the U.S. Pat. No. 8,173,513, which is incorporated by
reference herein in its entirety.
Present at least partially within the membrane 19 are first
piezoresistive sensing elements 28 (in particular, four in number,
set at the vertices of an ideal cross centered at the center of the
membrane 19), constituted by regions with a doping, for example, of
a P type. The first piezoresistive sensing elements 28 may be
obtained via diffusion of dopant atoms through an appropriate
diffusion mask, and have, for example, an approximately rectangular
cross-section. In addition, the first piezoresistive sensing
elements 28 may be connected together so as to form a
Wheatstone-bridge circuit.
Alternatively, the first piezoresistive sensing elements 28 may
form part of a ring oscillator circuit.
In a surface portion of the bulk region 17, in a position separate
and distinct from the membrane 19, second piezoresistive sensing
elements 29 are present (in particular, four in number, set at the
vertices of a further ideal cross centered at the center of the
membrane 19), which are also, for instance, formed by regions
having, for example, a doping of a P type obtained by diffusion.
The second piezoresistive sensing elements 29 are separated from
the membrane 19 (and thus from the first piezoresistive sensing
elements 28) by a distance (measured along the axis X) equal to or
greater than, for example, 10 .mu.m, preferably 50 .mu.m, so as not
to be affected significantly by the stresses on the membrane 19
(and on the first piezoresistive sensing elements 28) when the
force P acts. In particular, the second piezoresistive sensing
elements 29 are integrated in a solid and compact portion of the
bulk region 17, having a thickness substantially equal to the
distance w.
According to one embodiment, an interface layer 34 coats the first
surface 16a of the monolithic body 16. The interface layer 34 may
be a mono-layer or a multi-layer comprising an elastic material,
such as for example polyamide, or else may be a mono-layer or a
multi-layer of dielectric material, for example silicon oxide, or
alternatively a multi-layer comprising a silicon-oxide layer on
which a polyamide layer extends. The interface layer 34 may
comprise one or more metallization levels (not illustrated),
interconnected by connection vias.
A first substrate 32, for example of semiconductor material, such
as silicon, extends over the interface layer 34, coupled to the
interface layer 34 by anchorage elements 31 that extend between the
interface layer 34 and the first substrate 32, in peripheral
regions of the interface layer 34 and of the underlying monolithic
body 16. Considered herein as peripheral regions are those regions
of the monolithic body 16 that extend (when considered in the
planes XY, XZ and YZ) outside the second sensing elements 29. In
particular, the anchorage elements 31 extend along the entire
perimeter of the interface layer 34 and of the monolithic body 16,
outside the second piezoresistive sensing elements 29. The
anchorage elements 31 laterally surround and define a cavity 36
that extends between the first substrate 32 and the interface layer
34.
The anchorage elements 31 are made, for example, of dielectric
material, such as silicon oxide or silicon nitride, and are
obtained with deposition and etching steps, in themselves
known.
A first focusing region 30, which is also for example of dielectric
material, such as silicon oxide or silicon nitride, extends
between, and in direct contact with, the interface layer 34 and the
first substrate 32. In particular, the first focusing region 30
extends over the membrane 19, i.e., at least partially aligned to
the membrane 19 along the axis Z. In this way, in use, the first
focusing region 30 focuses the pressure P on the membrane 19
itself, forcing it to undergo deformation. The first focusing
region 30 is obtained during the same steps of production of the
anchorage elements 31.
The pressure sensor 15 further comprises a second substrate 35,
made, for example, of semiconductor material, such as silicon
(having a thickness comprised, for instance, between 50 and 900
.mu.m, preferably between 500 and 900 .mu.m, in particular 720
.mu.m), or ceramic material, or glass, or some other material
still, having a similar coefficient of elasticity, which extends
facing the second surface 16b of the monolithic body 16,
mechanically coupled to the second surface 16b by anchorage
elements 37 that extend between the second surface 16b of the
monolithic body 16 and a respective surface 35a of the second
substrate 35, at least in part in peripheral regions of the second
surface 16b of the monolithic body 16. As defined previously,
considered as peripheral regions are those regions of the
monolithic body that extend, as viewed in the planes XY, XZ, and
YZ, outside the second piezoresistive sensing elements 29. However,
in this case, as is on the other hand illustrated in FIG. 2, the
anchorage elements 37 extend in part, once again as viewed in the
plane XY, on top of the second piezoresistive sensing elements
29.
In particular, the anchorage elements 37 extend along the entire
perimeter of the second surface 16b of the monolithic body 16 and
of the surface 35a of the second substrate 35 so as to define a
second cavity 38 between the second surface 16b and the surface
35a.
The anchorage elements 37 are of dielectric material, for example
silicon oxide or silicon nitride, and have a thickness, along Z,
comprised for example between 0.1 .mu.m and 20 .mu.m, for example 1
.mu.m. The distance (along Z) between the second surface 16b and
the surface 35a defines the height of the second cavity 38,
substantially equal to the thickness of the anchorage elements
37.
Extending further between the interface layer 34 and the first
substrate 32 and in the cavity 36 are one or more second focusing
regions 33, which are coupled to the first substrate 32 (but not to
the interface layer 34) and have a thickness, along Z, smaller than
the thickness, once again along Z, of the first focusing region 30.
In particular, the second focusing regions 33 have a thickness such
that, when the membrane 19 comes into contact with the bottom of
the first cavity 18, or saturates (i.e., it is completely
deflected, thus closing the first cavity 18), or reaches the
desired full-scale value, the second focusing regions 33 are in
direct contact with the interface layer 34. In other words, the
distance (along Z) between the second focusing regions 33 and the
interface layer 34 is equal to or smaller than the thickness (along
Z) of the first cavity 18.
As an alternative to what has been described, the second focusing
regions 33 may be coupled to the interface layer 34 but not to the
first substrate 32, with which they come into direct contact when
the membrane 19 reaches the bottom of the cavity 18, or saturates,
or reaches the desired full-scale value.
According to a further embodiment, the second focusing regions 33
may be provided coupled in part to the interface layer 34 and in
part to the first substrate 32.
Thus, when a pressure P is applied in use on the pressure sensor,
the membrane 19 undergoes deflection until it comes into contact
with the bottom of the cavity 18. A minimum pressure value
P.sub.MAX1 for bringing the membrane 19 into contact with the
bottom of the cavity 18 depends upon the thickness of the membrane
19 and upon the material of which it is made. For instance, the
membrane 19 is produced in such a way as to come into contact with
the bottom of the cavity 18 when it is subjected at least to a
pressure P.sub.MAX1 comprised between 8 and 50 N. In particular,
with a membrane 19 of monocrystalline silicon having a thickness,
along Z, equal to 8 .mu.m, the pressure value P.sub.MAX1 is 10
N.
Intermediate pressure values P.sub.INT1<P.sub.MAX1 are such as
to cause progressive deflection of the membrane 19 (the higher the
current value of P.sub.INT1, the greater the deflection of the
membrane), but not such as to bring it into contact with the bottom
of the cavity 18.
As the pressure P increases, the second focusing regions 33,
together with the first focusing region 30, co-operate to bring
about deflection of the monolithic body 16, which thus behaves, as
a whole, as a second membrane suspended over the second cavity 38.
A minimum pressure value P.sub.MAX2 such as to bring the monolithic
body 16 into contact with the bottom 35a of the cavity 38 depends
upon the thickness of the monolithic body 16 and upon the material
of which it is made. For instance, the monolithic body 16 is
produced in such a way as to come into contact with the bottom 35a
of the cavity 38 when it undergoes a pressure at least equal to
P.sub.MAX2 higher than P.sub.MAX1 (and having a maximum value such
as not to damage the pressure sensor, for example a full-scale
value comprised between 10 and 20 kN). In a particular example,
with a monolithic body 16 of monocrystalline silicon having a
thickness w of 720 .mu.m, the pressure value P.sub.MAX2 is 10
kN.
Intermediate pressure values P.sub.INT2 such that
P.sub.MAX1<P.sub.INT2<P.sub.MAX2 are such as to cause
progressive deflection of the monolithic body 16 (the higher the
current value of P.sub.INT2, the greater the deflection of the
monolithic body 16), but not to bring the monolithic body 16 into
contact with the bottom 35a of the cavity 38.
The second piezoresistive sensing elements 29 have the function, in
use, of detecting the degree of deflection of the second membrane,
i.e., of the monolithic body 16, when the deflection of the first
membrane 19 is maximum (saturation condition). For this reason, it
is preferable to form the second piezoresistive sensing elements 29
sufficiently far from the first membrane 19 so that they will not
be affected by its deflection, but in any case in a region of the
monolithic body 16 that undergoes deflection when the first
membrane 19 is saturated. For instance, they may be set
substantially aligned, along Z, with respective peripheral regions
of the second cavity 38, i.e., regions of the second cavity 38
close to or bordering on the anchorage elements 37.
As has been anticipated, the general operation of the pressure
sensor 15 is based upon the so-called piezoresistive effect,
whereby a stress applied on a piezoresistive element causes a
variation of resistance thereof. In the case of semiconductor
materials, such as silicon, the stress applied, in addition to
determining a variation of the dimensions of the piezoresistive
element, brings about a deformation of the crystalline lattice and
thus an alteration of the mobility of the majority charge carriers
and a variation of resistivity. For instance, in silicon, to a
deformation of 1% of the crystalline lattice, there corresponds a
variation of approximately 30% of the mobility of the majority
charge carriers. In particular, the variation of resistance is
caused by stresses acting both in a parallel direction (so-called
longitudinal stresses) and in a direction normal to the plane in
which the piezoresistive elements lie (so-called transverse
stresses). The variation of resistance of a piezoresistive element
may in general be expressed by the following relation:
.DELTA..times..times..pi..times..sigma..sigma. ##EQU00001## where R
is the resistance of the piezoresistive element, .DELTA.R is the
variation of said resistance, .PI..sub.44 is one of the
piezoresistive coefficients of the semiconductor material, for
example equal to 138.110.sup.-11 Pa.sup.-1 for monocrystalline
silicon of a P type, and .sigma..sub.1, .sigma..sub.2 are,
respectively, the longitudinal stress and the transverse stress to
which the piezoresistive element is subjected.
With reference to the pressure sensor 15 of FIG. 2, the monolithic
body 16 is arranged in such a way that the pressure P to be
measured causes a stress in a direction normal to the first main
outer surface 16a (i.e., in this embodiment, along Z).
In particular, in a first operating condition, the pressure P
brings about a deformation of the membrane 19, which is forced to
undergo deformation. This deformation induces longitudinal and
transverse mechanical stresses in the first piezoresistive sensing
elements 28, which consequently modify the value of resistance.
Considering, for example, a Wheatstone-bridge configuration of the
first piezoresistive sensing elements 28, generally they are set in
such a way that part of them (e.g., two of them) undergo a
compressive stress, and the remaining ones (the other two, in the
case provided by way of example of four piezoresistors) undergo
tensile stress so as to increase the sensitivity of the
corresponding Wheatstone-bridge circuit. The variation of
resistance of the first piezoresistive sensing elements 28 thus
causes an unbalancing of the Wheatstone-bridge circuit, which
generates a voltage signal at output from the Wheatstone-bridge
circuit, which may be detected by an appropriate read circuit.
In addition, in a second operating condition in which the pressure
P assumes a value higher than the one required for bringing the
second focusing regions 33 into contact with the interface layer
34, a deformation of the monolithic body 16 is induced that brings
about a longitudinal and transverse mechanical stress in the second
piezoresistive sensing elements 29, which consequently modify the
value of resistance, as described with reference to the first
piezoresistive sensing elements 28.
In detail, one aspect of the present disclosure is based upon the
realization that for low values of the pressure P, the deformation
of the second piezoresistive sensing elements 29 is practically
negligible. Instead, the membrane 19 is induced to undergo
deformation, causing a corresponding deformation of the first
piezoresistive sensing elements 28, which is detected by the read
circuit in order to supply a measurement of the pressure P applied.
As the pressure P increases, the deformation of the membrane 19
increases until the membrane 19 itself comes into contact with the
bottom of the underlying first cavity 18, thus saturating the
pressure value supplied at output (in so far as any further
deformation is not possible). In particular, this saturation may
occur for values of the pressure P for example around 10 N.
At this point, a further increase in the pressure P begins to
affect the entire first main outer surface 16a and to cause a
deflection of the monolithic body 16, causing a consequent
non-negligible variation of the resistance of the second
piezoresistive sensing elements 29, from which the value of the
pressure P is derived. Saturation of the second membrane obtained
by the monolithic body 16 occurs for values of the pressure P
around 10 kN.
Consequently, the measurements of pressure supplied by the first
and second piezoresistive sensing elements 28, 29 are independent
and complementary, given that said elements intervene for different
values of the pressure P. The pressure sensor 15 thus has a first
measuring scale, valid for low values of the pressure P and a
full-scale value around 10 N (determined by the action of the
membrane 19 and of the first piezoresistive sensing elements 28,
which thus form together an element sensitive to low pressures),
and a second measuring scale, valid for high values of the pressure
P and having a full-scale value around 10 kN (determined by the
action of the monolithic body 16 and of the second piezoresistive
sensing elements 29, which thus form together an element sensitive
to high pressures). The first measuring scale is more precise than
the second, given that the membrane 19 is sensitive to even minimal
variations of the pressure P.
The pressure sensor 15 presents a considerable strength in regard
to high pressures. As is known, in fact, monocrystalline silicon
has a high ultimate strength in regard to compressive stresses, in
particular up to 2 GPa, according to the crystallographic
orientation, so that it is able to withstand, with ample margin,
the maximum pressure values that arise within a braking system.
Furthermore, the deflections of the membrane 19 in a vertical
direction are limited by the relatively small thickness of the
first cavity 18, thus preventing failure of the membrane 19 for
high pressure values.
The first piezoresistive sensing elements 28 may, for example, be
connected together to form a Wheatstone-bridge circuit (FIG. 3),
with resistors that vary in the same direction set on opposite
sides of the bridge so as to increase the sensitivity of the
circuit. The Wheatstone-bridge circuit is supplied with a supply
voltage V.sub.in1 and supplies an output voltage V.sub.out1.
The second piezoresistive sensing elements 29 may in turn be
connected so as to form, for example, an own Wheatstone-bridge
circuit, similar to the one illustrated in FIG. 3. Advantageously,
the particular arrangement of the piezoresistors in the
Wheatstone-bridge circuit enables a differential measurement to be
made, where the variations of resistance due to the environmental
parameters (for example, temperature) cancel out, thus rendering
the second output voltage V.sub.out1, and thus the value of the
pressure P measured, insensitive to said parameters.
Note, in particular, that the first and second piezoresistive
sensing elements 28, 29 are not electrically connected together and
form part of two distinct and independent electronic read circuits
(so as to provide, as highlighted previously, the two measuring
scales of the pressure sensor 15). In particular, for low values of
the pressure P, the voltage at output from the circuit formed by
the second piezoresistive elements 29 is substantially zero,
whereas the voltage V.sub.out1 at output from the circuit formed by
the first piezoresistive elements 28 is used by an appropriate
electronic measuring circuit (of a per se known type and
comprising, for example, at least one instrumentation amplifier)
for measuring the pressure P. Instead, for high values of the
pressure P, the output voltage V.sub.out1 of the circuit formed by
the first piezoresistive elements 28 saturates, and the electronic
measuring circuit obtains the measurement of the pressure P from
the output voltage V.sub.out2 of the circuit formed by the second
piezoresistive elements 29.
FIG. 4 shows a perspective view of a portion 15' of the pressure
sensor 15 of FIG. 2. In particular, the portion 15' represented in
FIG. 4 is a portion of pressure sensor 15 cut along the plane of
section of FIG. 2. Joining of two specular portions 15' forms the
pressure sensor 15.
As may be seen from FIG. 4, the anchorage elements 31 extend along
the entire perimeter of the monolithic body 16, forming a frame on
which the first substrate 32 rests. Likewise, the second focusing
regions 33 extend within the region defined by the anchorage
elements 31, mechanically isolated from the latter region so as to
be able to undergo deflection together with the first substrate
32.
In addition, the second focusing regions 33 have a recess within
which the first focusing region 30 is housed. The second focusing
regions 33 are thus also separate from the first focusing region 30
so as not to have constraints in order to undergo deflection
together with the first substrate 32. In the embodiment of FIG. 4,
the second focusing regions 33 extend joined to one another to form
a single region. However, according to different embodiments, they
may be mechanically separate/isolated from one another, for example
isolated from one another at the recess that houses the first
focusing region 30.
The anchorage elements 37 have a shape and extension similar to
that of the anchorage elements 31, and define the second cavity 38,
inside the frame formed by the anchorage elements 37.
According to a further embodiment (not illustrated in the figure),
the anchorage elements 31 and the second focusing regions 33 are
joined together, but with a respective thickness (along Z) that
varies along the axis X so that the anchorage elements 31 and the
second focusing regions 33 have a different thickness, as already
described.
FIG. 5 is a lateral sectional view of a further embodiment of a
pressure sensor 50, according to a further aspect of the present
disclosure.
The pressure sensor 50 comprises a base substrate 52, for example
of semiconductor material such as silicon, or ceramic material, or
glass, or some other material still, having a similar coefficient
of elasticity, mechanically coupled to a monolithic body 56 of
semiconductor material, such as silicon, by one or more anchorage
elements 54. The anchorage elements 54 are similar to the anchorage
elements 31 of FIGS. 2 and 4, and are not described any further
herein.
The monolithic body 56 is similar to the monolithic body 16
described previously, and houses a buried cavity 58. The buried
cavity 58 corresponds to the first cavity 18 of FIG. 2 and is
obtained with the same manufacturing method. Extending between the
buried cavity 58 and a surface 56a of the monolithic body 56 is a
flexible membrane 59 that is able to undergo deflection in the
presence of external loads. In particular, as already described in
detail with reference to the membrane 19 of FIG. 2, the membrane 59
undergoes deformation as a function of the pressure P acting on the
monolithic body 56.
Present at least partially within the membrane 59 are first
piezoresistive sensing elements 68, which are similar to the
piezoresistive elements 28 of FIG. 2 and have the same purpose. In
particular, the first piezoresistive sensing elements 68 are four
in number, are constituted by regions with a doping of a P type,
and are connected together so as to form a Wheatstone-bridge
circuit. In a per se known manner, the resistance of the first
piezoresistive sensing elements 68 is variable as a function of the
deformation of the membrane 59.
In a position separated and distinct from the membrane 59, second
piezoresistive sensing elements 69 are present, which are similar
to the piezoresistive elements 29 of FIG. 2 and have the same
purpose. These are also constituted by regions with a doping of a P
type and are separated from the membrane 59 by a distance such as
not to be affected significantly by the stresses acting on the
membrane 59 during a first operating condition of action of the
force P (e.g., up to 10 N). Hereinafter, the force P will refer
indifferently to a force or a pressure that the same force exerts
on a surface.
A first focusing region 66, similar to the first focusing region 30
of FIGS. 2 and 4, extends between the substrate 52 and the
monolithic body 56, in an area corresponding to the membrane
59.
Second focusing regions 73, similar to the second focusing regions
33 of FIG. 2, extend between the first focusing region 66 and the
anchorage elements 54. The second focusing regions 73 have a
thickness, along Z, smaller than the thickness, along Z, of the
anchorage elements 54 and of the first focusing region 66.
Advantageously, the second focusing regions 73 are already aligned
to the first and second piezoresistive sensing elements 68, 69.
Note that here, as likewise in the ensuing figures, for simplicity
of representation the interface layer 34 has been omitted.
In use, when the pressure P is applied, the presence of the first
focusing region 66 causes a deflection of the membrane 59, which
undergoes deflection in proportion to the pressure applied, until
it comes into contact with the internal wall of the cavity 58
(first operating condition); as the pressure P increases, the
monolithic body 56 undergoes deflection in the area corresponding
to the portions thereof that house the second focusing regions 73
(i.e., in the area corresponding to the portions of the monolithic
body 56 that extend between the anchorage elements 54 and the first
focusing region 66), until the second focusing regions 73 come into
contact with the substrate 52, thus determining a full-scale value
for the measurement of the deflection, and preventing undesirable
failure of or damage to the monolithic body 56 (second operating
condition). The monolithic body 56 thus behaves as a second
membrane suspended on the cavity present between the monolithic
body 56 itself and the substrate 52.
FIG. 6 shows a further embodiment of a pressure sensor 80 according
to the present disclosure.
The pressure sensor 80 comprises a monolithic body 82, for example
of semiconductor material (e.g., silicon). The monolithic body 82
extends between a first substrate 88 and a second substrate 84. In
greater detail, the monolithic body 82 is mechanically coupled to
the second substrate 84 by anchorage elements 85 similar to the
anchorage elements 37 described with reference to FIG. 1. Thus, the
anchorage elements 85 extend along a peripheral, or perimetral,
region of the monolithic body 82 and define a cavity 86. According
to one embodiment, the anchorage elements 85 surround the cavity 86
completely so that said cavity 86 is completely isolated from
outside. According to a different embodiment, the cavity 86 is only
partially surrounded by the anchorage elements 85. Set on the side
of the monolithic body 82 opposite to the side on which the
anchorage elements 85 extend is the first substrate 88, for example
of semiconductor material, similar to the first substrate 32
described with reference to FIG. 2, or ceramic material, or glass,
or some other material still, having a similar coefficient of
elasticity. In particular, the first substrate 88 is mechanically
coupled to the monolithic body 82 by anchorage elements 91 similar
to the anchorage elements 31 of FIG. 2. Furthermore, extending
between the first substrate 88 and the monolithic body 82 are first
and second focusing regions 90, 93 similar to the respective first
and second focusing regions 30, 31 of FIG. 2, and thus not
described any further. In a variant (not illustrated), the second
focusing regions 93 may be arranged on the monolithic body 82, like
the second focusing regions 73 of FIG. 5.
Piezoresistive sensing elements 94 (in particular four in number,
electrically connected together to form a Wheatstone-bridge
circuit), constituted by regions with a doping of a P type, extend
in the monolithic body 82 in the proximity of the surface thereof
facing the first substrate 88. More in particular, the
piezoresistive elements extend in a portion of the monolithic body
82 between the anchorage elements 91 and the second focusing
regions 93, specularly with respect to the first focusing region
90.
In use, during a first operating condition, a pressure, or force, P
is applied to the first substrate 88 and is transferred, by the
anchorage elements 91 and of the first focusing region 90, to the
monolithic body 82. The monolithic body 82 consequently undergoes
deflection, generating a longitudinal and transverse stress in the
area corresponding to the piezoresistive sensing elements 94. FIG.
7 shows, qualitatively, the plot of the output voltage signal
generated by the Wheatstone-bridge circuit during the first
operating condition (V.sub.out1) and during the second operating
condition (V.sub.out2).
If the force P applied is such as to bring the second focusing
regions 93 to come into contact with the monolithic body 82 (in the
example of FIG. 7, this event there corresponds to a pressure of 10
N and generates at output from the bridge circuit a voltage
V.sub.MAX1), the pressure sensor 80 enters a second operating
condition, where the monolithic body 82 continues to undergo
significant deflection in the area corresponding to the regions
comprised between the anchorage elements 91 and the second focusing
regions 93, i.e., in the portion thereof that houses the
piezoresistors 94. In the example of FIG. 7, at the pressure of 10
kN the second focusing regions 93 come into contact with the
monolithic body 82, and an output voltage V.sub.out2 is generated
by the bridge circuit equal to V.sub.MAX2>V.sub.MAX1.
As may be noted from FIG. 7, in the second operating condition, the
voltage V.sub.out2 at output from the Wheatstone-bridge circuit
changes slope with respect to the voltage V.sub.out1. Knowing the
plot of the signal V.sub.out at output from the bridge circuit
(which may be obtained experimentally in a per se known manner by
applying an increasing force P and measuring the output V.sub.out)
it is thus possible to identify, at each instant of operation of
the pressure sensor 80, in which operating condition it is by
correlating the output voltage value V.sub.out with the effective
pressure value P to which the pressure sensor is subjected.
FIG. 8 is a cross-sectional view of the pressure sensor 15 of FIG.
2 in which the second substrate 35 has an extension, in the plane
XY, greater than the respective extension of the monolithic body
16. In turn, the monolithic body 16 has an extension, in the plane
XY, greater than the respective extension of the first substrate
32. In this way, it is possible to provide electrical-contact pads
98 suitably connected (in a way known and not illustrated herein),
for example to the first and second piezoresistive sensing elements
28, 29 on the top surface 16a of the monolithic body 16 alongside
the first substrate 32. By way of example, only two pads 98 are
visible in FIG. 8, but they may be any in number, as required.
Between the pads 98 and the first and second piezoresistive sensing
elements 28, 29 a further circuit may be present, for example an
interface circuit, or an acquisition circuit, or a conversion
circuit so as to encode appropriately the value of the physical
quantity measured by the pressure sensor 15. It is likewise
possible to provide respective electrical-contact pads 99
(electrically coupled to conductive paths, not illustrated) on the
top surface 35a of the second substrate 35. By way of example, only
two pads 99 are visible in FIG. 8, but they may be any in number,
as desired.
The pads 98 are electrically connected to respective electrical
output terminals of the Wheatstone-bridge circuits formed,
respectively, by the first piezoresistive sensing elements 28 and
by the second piezoresistive sensing elements 29. The electrical
connection between the pads 98 and the pads 99 is obtained by wire
bonding 97. With the use of appropriate conductive paths coupled to
the pads 99 it is thus possible to transfer the electrical signal
supplied by the Wheatstone-bridge circuits outside the pressure
device 15, for example to the control unit 4 of FIG. 1. Further
electrical contact pads (not illustrated) may be provided for
sending the power supply to the pressure sensor.
FIG. 9 shows a further embodiment of a pressure sensor 101, where
the electrical signal generated by the first and second
piezoresistive sensing elements 28, 29 is transferred outside the
pressure sensor 101 by inductive coupling.
In this embodiment, one or more inductors 102 are integrated in the
interface layer 34 (or, alternatively, in the monolithic body 16 on
the surface 16a), and respective one or more inductors 104 are
integrated in the first substrate 88 in such a way that each
inductor 102 is inductively coupled to a respective inductor 104.
In the case where first substrate 88 is made of semiconductor
material, it will be necessary for the respective one or more
inductors 104 to be integrated in the interface layer (here not
shown and similar to the layer 34). The inductors 102 are
operatively coupled to respective output terminals of circuits (for
example, they may comprise a transceiver/transponder, an AC-DC
converter, a finite-state digital circuit, a microcontroller), here
not illustrated, which comprise or are coupled to the first and
second piezoresistive sensing elements 28, 29, for example
connected via a Wheatstone bridge or forming part of a ring
oscillator circuit so as to receive the voltage signals generated
as a result of the deformation of the membrane 19 and of the
monolithic body 16, during use of the pressure sensor, and transfer
said signals to the respective inductors 104. The inductors 104 are
coupled to conductive paths (not illustrated) to transfer the
signal of detection of the pressure P outside the pressure sensor,
for example to the control unit 4 of FIG. 1. The pressure sensor
may be supplied via further electrical contact pads (not
illustrated) or via the inductors 102, 104 in a known way.
According to a further embodiment of a pressure sensor 105
(illustrated in FIG. 10), one or more inductors 106 are integrated
in the interface layer 34. However, the first substrate 88 does not
integrate respective inductors. Inductors 109 are instead provided
in an external board, for example a PCB (printed-circuit board)
110. During mechanical coupling of the pressure sensor 105 of FIG.
10 to the PCB 110, the pressure sensor 105 is set on the PCB 110 in
such a way that the inductors 106 are inductively coupled, in use,
to the inductors 109. The inductors 106 and 109 are appropriately
sized in order to guarantee inductive coupling, and the substrate
88 extending between them should preferably have a high resistivity
(for example, it may be intrinsic silicon or a dielectric material
such as a ceramic or glass) so as to prevent onset of eddy
currents. For instance, the inductors 109 are larger in size than
the inductors 106.
Owing to the presence of the PCB 110, the pressure P is applied, in
this example, on the second substrate 35.
Appropriate electrical connections are provided, in a per se known
manner, on the PCB 110, for acquiring an electrical signal from the
inductors 109 and sending it, for example, to the control unit 4 of
FIG. 1 to be processed. The pressure sensor may be supplied via
further electrical contact pads (not illustrated) or via the
inductors 106, 109 in a known way.
FIG. 11 shows, in top view, a further embodiment of a pressure
sensor 115 where the first substrate 32 has an extension, in the
plane XY, smaller than the respective extension of the monolithic
body 16. In particular, the first substrate 32 is here modelled in
such a way as to expose selective portions of the surface of the
interface layer 34 that extends over the monolithic body 16. The
exposed selective portions are corner regions of the interface
layer 34 that extends over the monolithic body 16 (here assumed as
being quadrangular, in particular square). Extending in the area
corresponding to the exposed regions of the interface layer 34 is a
plurality of contact pads 118 of conductive material, designed to
be electrically contacted, for example by metal strips 119 (e.g.,
copper strips). The contact pads 118 are in turn in electrical
contact with respective terminals of the circuits that comprise the
first and second piezoresistive elements 28 and 29 for acquiring
the signal transduced by them. Further electrical contact pads (not
illustrated) may be provided for sending the power supply to the
pressure sensor.
FIG. 12 shows a further embodiment of a pressure sensor 120. The
pressure sensor 120 of FIG. 12 is similar to the pressure sensor 50
of FIG. 5. Elements of the pressure sensor 120 and of the pressure
sensor 50 that are in common are here not described or illustrated
any further and are designated by the same reference numbers.
The pressure sensor 120 further comprises a first blade connector
122 and a second blade connector 124 (also known as "fastons"); the
first blade connector 122 extends over the exposed side of the
monolithic body 56, whereas the second blade connector 124 extends
over the exposed side of the base substrate 52. In this way, the
monolithic body 56 and the base substrate 52 are sandwiched between
the first and second blade connectors 122, 124.
The first and second blade connectors 122, 124, which are made of
conductive material, in particular metal, have the function of
sending the power supply to the pressure sensor 120. For instance,
the first blade connector 122 is biased at a supply voltage
V.sub.DD, whereas the second blade connector 124 is biased at a
reference voltage, for example the ground voltage GND. For this
purpose, in the coupling regions between the monolithic body 56 and
the first blade connector 122, and in the coupling regions between
the base substrate 52 and the second blade connector 124, there
extend respective electrical contact pads (not illustrated). In
order to connect the second blade connector 124 electrically to the
circuits (not illustrated, which comprise or are connected to the
piezoresistive sensing elements 68, 69) in the monolithic body 56,
it is necessary for at least one portion (for example, the outer
ring) of the anchorage elements 54 to be conductive. Furthermore,
the base substrate 52 and the monolithic body 56 must present a low
resistivity.
According to one embodiment of the present disclosure, the first
and second blade connectors 122, 124 further have the function of
sending the signals transduced by the piezoresistive sensing
elements 68, 69 outside the pressure sensor 120, for example to the
control unit 4 of FIG. 1. In this case, according to techniques in
themselves known, the electrical carrier signal (supply signal) is
modulated in such a way as to function as data-carrying medium, for
carrying also the signal transduced by the piezoresistive elements
68, 69 (signal at output, for example, from the respective
Wheatstone-bridge circuits), or else said supply signal may be a
constant voltage, superimposed then on which is a digital signal
that carries the information of the physical quantity measured.
The pressure sensor described, according to the respective
embodiments, presents numerous advantages.
In the first place, it presents a high full-scale value and enables
measurement of pressures with a double measuring scale, a first
scale for measuring low pressures, and a second scale for measuring
high pressures. Both measurements are made with high precision. In
particular, the pressure sensor described integrates within a same
monolithic body of semiconductor material the elements sensitive to
high and low pressures, with contained costs and limited complexity
of production.
The pressure sensor makes a measurement of a differential type
between one or more sensing elements and one or more piezoresistive
reference elements and is consequently insensitive to variations of
environmental parameters or to process spread.
Finally, it is clear that modifications and variations may be made
to what has been described and illustrated herein, without thereby
departing from the scope of the present disclosure.
In particular, it is clear how the shape of the monolithic body may
be different from what has been described and illustrated. In
particular, the cross-section of the monolithic body may be
circular or generally polygonal, instead of being quadrangular or
square as described. Also the first cavity 18 may have a shape
different from what has been illustrated, for example a circular or
generically polygonal cross-section. Likewise, also the second
cavity 38 may have a shape different from what has been
illustrated, for example a circular or generically polygonal
cross-section.
Irrespective of the particular embodiment described, the
piezoresistive sensor elements could be obtained using
ion-implantation techniques, instead of by diffusion.
Irrespective of the particular embodiment described, it is possible
to use electrical connections, between the piezoresistive elements,
different from the Wheatstone bridge, for example a ring oscillator
circuit, or some other connection still.
It is likewise possible to use a single piezoresistive sensing
element for detecting deflections of the first membrane during the
first operating condition, and a further single piezoresistive
sensing element for detecting deflections of the second membrane
(monolithic body) during the second operating condition of the
pressure sensor.
Irrespective of the particular embodiment described, further, the
piezoresistive elements may be located in a position different from
the one illustrated.
Irrespective of the particular embodiment described, it is further
possible to form, for example within the first substrate or the
second substrate, an electronic measuring circuit, so as to provide
a pressure measuring device integrated in a single die.
In addition, the focusing regions and the anchorage regions could
be obtained also using non-dielectric material, for example
conductive material or semiconductor material.
Furthermore, using a semiconductor material different from silicon,
for example gallium arsenide, the sensing elements could have
piezoelectric characteristics instead of piezoresistive ones.
Finally, it is pointed out that the pressure sensor 15 may
advantageously be used also in other applications to measure high
pressure values with a double measuring scale.
The various embodiments described above can be combined to provide
further embodiments. These and other changes can be made to the
embodiments in light of the above-detailed description. In general,
in the following claims, the terms used should not be construed to
limit the claims to the specific embodiments disclosed in the
specification and the claims, but should be construed to include
all possible embodiments along with the full scope of equivalents
to which such claims are entitled. Accordingly, the claims are not
limited by the disclosure.
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